This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Serine/threonine phosphatase 5 (PP5) is an enzyme that has an under appreciated role in the regulation of signal transduction and altered PP5 activity appears to contribute to tumor development and maintenance. While protein kinase enzymes catalyze the transfer of a phosphate group from a donor to an acceptor protein in various signaling pathways, protein phosphatases, in turn, reverse the action of kinases by removing a phosphate. This removal may activate or inactivate the acceptor protein. Both genetic studies and studies using inhibitors of protein phosphatases (e.g. okadaic acid) indicate protein phosphatases play an important role(s) in the regulation of cell cycle progression and related processes implicated in tumor promotion. The endogenous expression of PP5 is responsive to 17-beta estradiol and hypoxia inducible factor-1 (HIF1) which are both positive factors in the development of human breast cancer. Consequently, the constitutive over expression of PP5 converts MCF-7 breast cancer cells from an estrogen-dependent (standard state) into an estrogen-independent phenotype (increased malignancy potential). Thus, altered PP5 activity may contribute to tumor development. Studies in progress indicate that over expression of PP5 via an expression plasmid in human tissue culture cells also promotes survival during oxidative stress (low oxygen state common to tumors regulated in part by HIF1). To continue these studies we are constructing PP5 expression plasmids with altered catalytic domains to test the hypothesis that PP5 has a role in the regulation of hypoxic stress, and that changes in the normal biological functions of PP5 may contribute to the development of cancer.